Watermarks have been integrated into documents to verify authenticity since at least as early as the 1200's. The concept was to apply a unique, hard-to-replicate design feature that could quickly be identified by a stakeholder. This type of approach was applied in U.S. Pat. No. 353,666 (Crane, Jr.), entitled “Watermarked Paper” and filed in 1886, which notes that “when the paper thus produced is examined against the light”, unique features can be observed.
Photoluminescence (PL) is the emission of light (electromagnetic radiation, photons) after the absorption of light. It is one form of luminescence (light emission) and is initiated by photoexcitation (excitation by photons). Following photon excitation, various charge relaxation processes can occur in which other photons with a lower energy are re-radiated on some time scale. The energy difference between the absorbed photons and the emitted photons, also known as Stokes shift, can vary widely across materials from nearly zero to 1 eV or more. Time periods between absorption and emission may also vary, and may range from the short femtosecond-regime (for emissions involving free-carrier plasma in inorganic semiconductors) up to milliseconds (for phosphorescent processes in molecular systems). Under special circumstances, delay of emission may even span to minutes or hours. Further, for a given material or mixture of materials, the emission lifetime can depend on the excitation and emission wavelength.
Some uses of luminescent security inks for authentication are known to the art. This may be appreciated, for example, with respect to U.S. Pat. No. 2,742,631 (Rajchman et al.), entitled “Methods For Recording And Transmitting Information Using Phosphors”, which was filed in 1954, and U.S. Pat. No. 3,614,430 (Berler), entitled “Fluorescent-Ink-Inprinted Coded Document And Method And Apparatus For Use In Connection Therewith”, which was filed in 1969.